When an automobile driver travels long distances through rain, the driver must continually adjust the speed of the windshield wipers, because the intensity of the rain striking the windshield continually varies. Variation is caused by the natural variation in rain over time and distance, and also by man-made agents, such as passing vehicles, which shower nearby automobiles with spray.
This continual adjustment can be tedious, and rain sensors have been developed, in order to automate control of the wipers. FIG. 1 is a schematic view of a WINDSHIELD, and illustrates one type of sensor. The swaths S represent the fields swept by the wiper blades.
The sensor takes the form of parallel conductors C. When water bridges the gap G between the conductors, the electrical resistance of the gap G changes. A resistance sensor (not shown) detects this change, infers the presence of water, and calls for a wiping stroke.
This type of sensor presents some disadvantages. One is cost of manufacture. The long, finger-like conductors C are fabricated in a sequence of multiple steps which include masking, screening, and firing processes. The multiplicity of steps imposes cost.
Also, the conductor sections which are located outside the swaths S, such as at region R, must be protected from the rainwater, because the wipers do not remove the water in this region. Consequently, insulation is added to the conductors in this region, to insulate them from the water. Without insulation, the sensors would continually detect rain, and continually call for wiping strokes.
One solution to this particular problem is capacitive coupling, as described in U.S. Pat. No. 4,827,198. However, such coupling is not feasible in some windshields which bear a metallic film on the interior surface. Such films are used as resistance heaters, in order to de-frost the windshield.